Remark 1.1  Under the assumption of Theorem REF  we see that \(p\)  has necessarily non-real characteristic roots in the \(t<0\)  side near \((0,0,\xi )\) , that is \(P\)  would be a Tricomi type operator. In fact from [1]} it follows that when \(\tau \)  is a triple characteristic root then \(F_p(0,0,\tau ,\xi )=O\)  if all characteristic roots are real in a full neighborhood of \((0,0,\xi )\) .

